Rotating synchronous electric machines are used in various areas of society. For example, power production generators and motors intended for certain applications are usually of this type.
Electric power intended for distribution is generated by means of generators and transformed with the aid of transformers before being transferred and distributed via power grids at different voltage levels. Transformers and power grids are dependent on reactive power to be able to transform and transport the active power from the generators to the users. The reactive power is also used to control the voltage in the power systems. The active power that is generated is consumed in different ways by the users. Many electric appliances used in industry, offices and homes also consume reactive power, for example appliances equipped with a motor. To avoid excessive power loss in the distribution grid, it is necessary to generate reactive power or compensate for the consumption of reactive power. In prior art, this has been achieved, for example, by means of generators and shunt capacitors.
The maximum power produced/consumed in an electric machine is usually based on the machine rating, which is valid for the machine under specific operating conditions. The machine rating defines the maximum current that can flow through the windings of the machine without jeopardizing the service life or functioning of the machine.
Due to dissipation of energy, electric machines heat up during operation. The heat generated must be carried off by means of cooling to avoid overheating of the machine. Different coolants and cooling systems are used depending on the size, application and design of the machine. Cooling by air is predominant in the case of small machines. For enclosed air-cooled machines a heat exchanger is often provided, in which a coolant, such as water, cools the air circulating in and cooling the machine. The cooling systems become more sophisticated and complex with increasing size. Large power production generators are often direct water-cooled and/or indirect water-cooled. Direct water-cooling is used, for example, for rotor and stator alike, but combinations of water/air and water/hydrogen gas are common, the air/hydrogen gas combination being cooled by water before it in turn cools the generator. The coolant or coolants of the machine may in turn be cooled, via heat exchangers, by water from any primary coolant source such as a cooling tower, a river, a lake or the sea. Heat exchangers may, for instance, be of the coolant/water, coolant/air or water/hydrogen gas type. Cooling is achieved by supplying a coolant from the primary coolant source to heat exchangers on the primary side, while the secondary side is connected to a cooling circuit in the electric machine. Alternatively, an intermediate cooling circuit may be used, which may then be used also to cool other systems, such as the excitation system.
Due to disturbances in the normal operation of the power grid, there is sometimes a need for rapidly accessible and controllable reactive power. To meet this need synchronous machines are dimensioned to produce a certain reactive power output. In Sweden, Svenska Kraftnat (the Swedish National Grid) requires generators that are directly or indirectly connected to the national grid to be capable of continuously producing a reactive power output corresponding to a third of the active power output. This means that the generators most of the time will operate below name-plate rating, i.e. the machines are operated using a stator current that is lower than the maximum allowable current and a rotor current that is lower than the maximum allowable current, which results in the temperature of the machines being lower than the rated duty temperature.
Thus, during the greater part of the operating time there is surplus capacity in the generator. It would be desirable to be able to combine the reactive power output capability with normal operation at a more optimal operating point. It would also be desirable to be able to meet acute needs for higher reactive power output without having unexploited capacity in the generator during the greater part of the operating time.
U.S. Pat. No. 5,321,308 discloses a control method and a control apparatus for a generator. The specification describes how to maximize the production of reactive power of a generator. The increase in reactive power necessitates, however, a simultaneous reduction in the production of active power.
In “Dynamic Thermal Ratings Realize Circuit Load Limits”, IEEE Computer Applications in Power, pp 38-43, January 2000, a way of temporarily exploiting the power distribution, system better is described. However, the document makes no reference to rotating machines and says nothing about how to compensate for temporary disturbances in the power grid, which require increased production of reactive power.
Thus, it would be desirable to be able to compensate for temporary needs for more reactive power without affecting the available active-power. It is also desirable to be able to use the synchronous electric machine more efficiently without noticeably affecting its service life. It is further desirable to optimize the efficiency of existing and new synchronous electric machines.